The invention relates to an optical system having a light source and an objective having a lens of crystal.
Systems of the above kind have long been known as UV optical systems with calcium fluoride lenses as well as with barium fluoride lenses. These systems are also configured as microlithographic projection exposure systems having the highest requirements. Especially important wavelengths below 200 nm to the vicinity of 100 nm (that is, in the VUV range) become accessible via various fluorides. Examples of the above are provided in U.S. patent application Ser. No. 09/451,505, filed Nov. 30, 1999, (corresponding to German patent application DE 199 29 701.0) which is incorporated herein by reference.
Magnesium fluoride can be produced in large pieces and can be well processed with optical quality and combines high transmission to the lower limit of the above-mentioned range with good radiation resistance. However, magnesium fluoride was up to now not considered as a lens material because of its double refraction, as were all other double-refracting crystals.
VUV-optics and especially microlithographic projection objectives are already configured as catadioptric systems as disclosed, for example, in U.S. Pat. No. 5,815,310. This publication also shows a need for catadioptric systems when a suitable lens material is available as is the case for 193 nm.
It is an object of the invention to provide alternate configurations of VUV optical systems.
The optical system of the invention includes: a light source for generating light; and, an objective having lenses made of crystal and being mounted downstream of the light source for receiving the light which passes through the objective with an ordinary ray and an extraordinary ray; at least one of the lenses being made of a crystal having a dispersion of double refraction; the dispersion being characterized for the one lens by dispersion curves for the ordinary and extraordinary rays, respectively; the ordinary and extraordinary rays intersecting at an isotropic wavelength; and, the light being generated by the light source at the isotropic wavelength.
Reference is made to an article of V. Chandrasekharan et al entitled xe2x80x9cAnomalous Dispersion of Birefringence of Sapphire and Magnesium Fluoride in the Vacuum Ultravioletxe2x80x9d published in Applied Optics, volume 8, No. 3 (March 1969), pages 671 to 675. With this anomalous dispersion of birefringence, the refraction difference vanishes for MgF2 at 119.4 nm and for sapphire at 142.6 nm.
The invention makes use of this rather dated information and provides, for the first time, an imaging optical system wherein the double refraction of the lens material is eliminated by the selection of the light wavelength. In this way, the requirement of the microlithography for optical systems having VUV wavelengths is taken into account.
According to a feature of the invention, a Lyot filter is mounted in the beam path between the light source and the object.
The Lyot filter can be made of the same crystal as the lens. This feature of the invention ensures the suitable filtering of the light.
Optical lithography between 157 nm and 100 nm would be a very economical step for many who work in the area of lithography. The problems below 157 nm are primarily material problems. The main candidates for short wavelengths are presented in Table 1.
MgF2 is, however, very intensely double refracting. Even small parts of a millimeter in transmission are sufficient, depending upon the orientation and polarization condition, to cause an impermissible wave front split in the image plane.
A very unusual performance results with MgF2, especially when looking at the trace of the double refraction as a function of wavelength. With increasing radiation frequency, the double refraction increases continuously and reaches a maximum at approximately 153 nm. Thereafter, the double refraction drops very sharply and finally reaches negative values (see the above-mentioned article from Applied Optics). According to the invention, the point in the frequency is used at which the material is isotropic. This is the case at 119.49 nm.
The jump from 157 nm to 120 nm would be especially important for a further generation of microlithographic projection exposure systems. The double refraction is a function of the wavelength and the material temperature. For this reason, work ranges must be maintained for the isotropic point. The operation with the temperature is more likely easier to adjust because of the excellent thermal conductivity of MgF2 and the excellent temperature equalization in modern microlithographic projection objectives. More difficulty is experienced with the chromatic bandwidth of the objective made of MgF2. The chromatic bandwidth cannot be achromatized with respect to the double refraction. The dispersion of the double refraction at 119.49 nm is 0.256xc2x710xe2x88x926/pm=xcex94n. For a numerical aperture of NA=0.80, a xcex94n of approximately xc2x11.5xc2x710xe2x88x927=xcex94n is permitted because this is not a defect which can be focused upon. The stress-induced birefringence caused bandwidth of the system at NA=0.80 lies approximately at 0.5 pm. In this way, the birefringence induced bandwidth is greater than a pure isotropic dispersion bandwidth which one must set at approximately 0.1 nm for a full-field objective having NA=0.80 and MgF2 at 119 nm. With this, the following possibilities are provided by the invention:
1. a purely refractive, noncolor corrected objective with MgF2 at 119.49 nm and 0.1 pm laser bandwidth;
2. a catadioptric objective with MgF2 having a bandwidth of approximately 0.5 pm such as an objective according to Schwarzschild or an h-design, for example, as disclosed in U.S. Pat. No. 6,496,306, incorporated herein by reference;
3. a partly achromatized refractive objective having a bandwidth of approximately 0.5 pm, for example, BeF2+MgF2 or LiF+MgF2.
4. MgF2 is, at 119.49 nm, specifically orientated in the crystal direction and is irradiated with light of a special polarization. For example, tangential polarization via the pupil of the objective, E-vector of the MgF2 crystal parallel to the optical axis. In this way, the influence of the double refraction is mitigated for specific lenses close to the pupil and, for a telecentric embodiment, lenses close to the object and close to the image.
Broadband systems are thereby possible with, for example, a 1 to 2 pm bandwidth.
In the configuration of a laser for 119.49 nm, an additional effect can be used which narrows the bandwidth of the laser. Prisms, gratings and etalons function to narrow the bandwidth. It is possible to carry out a periodic extraction with the aid of a Lyot filter from the spectrum of rare gases and their excimer compounds.
Crossed polarizers and an MgF2 rod define the Lyot filter. In the MgF2 rod, the crystal axis is perpendicular to the longitudinal dimension of the rod and is at 45xc2x0 to the polarization axes. The modulation as a function of frequency assumes a known course but exhibits a distinctiveness at 119.49 nm when the rod is selected correspondingly long. At 119.49 nm, a wider transmissibility is exhibited as a function of frequency.
The temperatures of the objective and of the reference rod have to be coincident or have to be precisely measured and the bandpass range, which is used, has to be determined so that the arrangement works as well as possible. Thereafter, gratings, prisms and etalons can be ideally adjusted in order to adjust the ideal operating mode for the objective having a low phase delay. The Lyot filter can then either be in the active part of the light source or function as a wavelength reference.